List

A List<T> is a .NET data structure that is held in memory. With the data structures provided by the Couchbase .NET SDK, you can store it in a Couchbase document.

To create a Couchbase-backed List:

var list = new CouchbaseList<dynamic>(bucket, "myList");

The string "myList" corresponds to the key for the document that will contain the list. When using CouchbaseList<T>, a single document with that key will be created (if one doesn’t exist already). If a document by that key already exists, CouchbaseList will use it.

You can now add/remove items from the list and that will all be persisted to the document. You can also perform other operations like getting a count of the items in the list.

The above code would result in a document with a key "myList" that looks like below. Notice that the item with num of 5 is not listed, because it was removed.

There’s something subtle in the above example that needs to be pointed out. Notice that I used var item = list[5]; and then item.foo and item.num in the WriteLine. If I used list[5].foo and list[5].num directly, that would result in two different subdocument calls to Couchbase. Not only is this less than optimal efficiency, but it’s possible for the values to change between the two calls.

A note about C# dynamic: I used dynamic to keep the code samples short and simple. In your application, you are probably better off using a real defined C# type. It all gets serialized to JSON in Couchbase, of course.

Behind the scenes

Before the subdocument API was released in Couchbase Server 4.5, these data structures were possible, of course. The catch was that you would be loading up the entire document, putting it in a list, making changes to the list, and then saving the entire document. If you have large data structures, but are only reading or making changes to a single item, this would often result in wasted time and wasted bandwidth and possibly increased contention.

The subdocument-API (which you can use directly; I covered it in the Sub-document API in Couchbase Server 4.5 with the .NET SDK (revisited) blog post) is used behind the scenes in CouchbaseList, CouchbaseQueue, and CouchbaseDictionary. So when you add an item to a CouchbaseList, for instance, only that item is being sent over the wire, not the entire list.

Some operations will still need to get the entire document. For instance, iterating through a collection using a foreach loop will retrieve the full document. Removing an item from a list will result in the full document being scanned. But if sub-document operations come along in the future to support those actions, the SDK implementations will be updated accordinging.

Summary

These data structures are another tool to help you manage your data. Since they use the sub-document API, they are generally more performant than a whole-document approach. For more detail, check out the Data Structures documentation.